This invention relates to an antilock modulator for use in an automotive brake system.
FIG. 5 shows a conventional circulation type antilock device comprising a reservoir 2 for storing brake fluid discharged from a wheel brake 1 during antilock control, a single or a plurality of pairs of electromagnetic control valves 4, 5 for controlling the brake pressure on the wheel during antilock by selectively connecting the wheel brake 1 to a main fluid line 9 leading to a master cylinder 3 or a discharge line 10 leading to a reservoir 2 in response to control signals, and a hydraulic pump 6 for sucking and pressurizing the brake fluid in the reservoir 2 to return it to a fluid return point of the main fluid line.
The circulation type antilock device shown in FIG. 5 has a problem in that the hydraulic fluid discharged from the wheel brake 1 while the brake pressure is being reduced during antilock control mode is returned toward the master cylinder by the pump 6, so that a brake pedal 7 and the pipings tend to vibrate due to the pulsation of the pump, thus producing much noise.
An antilock device which is free of this problem is proposed e.g. in Examined Japanese Patent Publication 61-33738. This device has a first check valve provided in the main fluid line between the fluid return point and the master cylinder to allow only a fluid flow from the master cylinder toward the control valves, a second check valve provided in a bypass line connecting the wheel brake to the master cylinder to allow only a fluid flow toward the master cylinder, and an accumulator provided at the outlet side of pump. Since the brake fluid discharged from the pump is accumulated in the accumulator, the pump pulsation decreases.
But for this device, a rather large accumulator is needed, so that the entire device tends to be bulky and costly. Also, since the wheel brake pressure never drops below the pressure necessary to open the second check valve, brake dragging will occur.
Examined Japanese Patent Publication 61-54619 discloses an improved arrangement in which a pressure-adjusting valve is used in place of the accumulator, which acts to discharge pressure fluid pumped out from the hydraulic pump into the reservoir when the pressure of the pumped-out fluid becomes higher by a predetermined value than the fluid pressure produced in the master cylinder.
An antilock device proposed in Examined Japanese Patent Publication 61-16656 employs, in place of the above-described first check valve, a valve device which permits a free fluid flow in both directions while the fluid pressure in the master cylinder is below a predetermined value and permits only a fluid flow from the master cylinder toward the control valves when it exceeds the predetermined value.
But the device of Japanese Examined Patent Publication 61-54619 has the same problem with the device of Japanese Examined Patent Publication 61-33738, i.e. the problem in that the brake pressure never drops below the valve-opening pressure of the second check valve. The device disclosed in Japanese Examined Patent Publication 61-16656 has a problem in that the piston moves during normal braking operations, thus increasing the brake pedal stroke.
Another conceivable arrangement is to use a solenoid valve as the fluid pressure control valve while completely cutting off the fluid flow toward the master cylinder. In this case, if the amount of brake fluid necessary to sharply increase the wheel brake pressure exceeds the amount of the fluid discharged from the pump, the remaining portion of the fluid necessary to increase the brake pressure is supplied from the master cylinder, so that the brake pedal will be pushed in abruptly. On the other hand, while the brake pressure is being reduced or maintained, the pedal position will be constant. In other words, the brake pedal will be alternately stopped and advanced. This worsens the pedal feel.